Method for Manufacturing a Separation Membrane Based on a Polar Carbon Nanotube Dispersion and a Polar One-Dimensional Carbon Body

ABSTRACT

Provided are a polar carbon nanotube dispersion which may be dispersed in a solvent at a high concentration, and a separator having improved filtration efficiency based on a polar carbon nanotube manufactured from the dispersion and a polar one-dimensional carbon body. According to the separator and the method for manufacturing the same of the present invention, a polar carbon nanotube dispersion which may be dispersed in a solvent at a high concentration even without use of a surfactant or a stabilizer may be prepared, and a separator which is not easily exfoliated and may be stably used even under a high pressure may be manufactured, based on a polar carbon nanotube prepared from the polar carbon nanotube dispersion and a polar one-dimensional carbon body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Patent Application Nos.10-2021-0070527 filed Jun. 1, 2021 and 10-2022-0065583 filed May 27,2022, the disclosures of which are hereby incorporated by reference intheir entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The following disclosure relates to a polar carbon nanotube dispersionwhich may be dispersed in a solvent at a high concentration, and aseparator having improved filtration efficiency based on a polar carbonnanotube manufactured from the dispersion and a polar one-dimensionalcarbon body.

Description of Related Art

Since membrane separation is less expensive, has simpler equipment, andhas higher energy efficiency than other methods such as distillation andadsorption, it is effective for separating smallmolecules/ions/solvents. In recent years, two-dimensional materials suchas graphene oxide, transition metal dichalcogenide, and MXene are usedin membrane manufacture, due to their advantages of high solventpermeability and precise molecular selectivity. Since among various 2Dmaterials, graphene may allow manufacture of a large area by continuouscoating such as doctor blade and slot-die coating and may withstandchemical and mechanical conditions, it is selected as a particularlypromising material. As described above, graphene has many advantages,but is easily exfoliated depending on the conditions such as pressureand solvent, and thus, an additional treatment is needed for improvingmechanical stability and adhesive strength. Therefore, a support with anappropriate pore structure, stable mechanical/chemical properties, andstrong adhesive strength with a graphene layer is needed. In order tocomplement the support, various studies on polydopamine (PDA), carbonnanotubes (CNT), diamine, polyethyleneimine (PEI), plasma treatment, andthe like, have been conducted. However, although the chemicalcrosslinking between a support and graphene is effective for improvingstability of a membrane, the polymer chain in a polymer support may bebroken by a chemical treatment. In addition, an inorganic support havinghigh mechanical stability may be used even after surface modificationbut has a problem in terms of high manufacturing costs and brittleness.

Therefore, research and development of appropriate support are stillrequired for use of a graphene-based membrane under actual conditions.

SUMMARY OF THE INVENTION

An embodiment of the present invention is directed to providing a polarcarbon nanotube dispersion which may be dispersed in a solvent at a highconcentration, and a separator including a polar carbon nanotubemanufactured from the dispersion and a polar one-dimensional carbonbody, which may be stably used even at high pressure and has improvedfiltration efficiency.

In one general aspect, a separator includes: a porous support; a porousfirst coating layer including a polar carbon nanotube, placed on theporous support; and a second coating layer including a polarone-dimensional carbon body, placed on the porous first coating layer.

In the separator of the present invention, the polar carbon nanotube mayhave an oxygen/carbon atomic ratio of 0.1 or more and a full width athalf maximum of 2.0 or more at a position of 2θ=25.6° of XRD, and apolar group of the polar carbon nanotube may be positioned on thesurface of the carbon nanotube.

In the separator of the present invention, the porous first coatinglayer may be prepared by applying a viscoelastic polar carbon nanotubedispersion.

In the separator of the present invention, a polar group of the polarone-dimensional carbon body may include any one or more selected fromthe group consisting of a hydroxyl group, an epoxy group, and a carboxylgroup, the polar group may be unevenly distributed at the edge of thepolar one-dimensional carbon body, and the polar one-dimensional carbonbody may be graphene nanoribbon.

In the separator of the present invention, an interlayer spacing of thesecond coating layer may be 5 to 20 Å, a thickness ratio between theporous first coating layer and the second coating layer may be 5:1 to20:1, and the polar carbon nanotube may have a lower oxygen/carbonatomic ratio than the polar one-dimensional carbon body.

In the separator of the present invention, an organic dye rejection ratemay be 80% or more and a sodium chloride rejection rate may be 20% orless, with respect to a mixture including sodium chloride and an organicdye, under the condition of 10 bar.

In another general aspect, a method for manufacturing a separatorincludes: (a) preparing a dispersion including a polar carbon nanotube;(b) preparing a dispersion including a polar one-dimensional carbonbody; (c) applying the dispersion including a polar carbon nanotube on aporous support to prepare a porous first coating layer; and (d) applyingthe dispersion including a polar one-dimensional carbon body on theporous first coating layer to prepare a second coating layer.

In the method for manufacturing a separator of the present invention,the step (a) of preparing a dispersion including a polar carbon nanotubemay include (a-1) partially oxidizing a carbon nanotube to prepare thepolar carbon nanotube; (a-2) mechanically milling the polar carbonnanotube; and (a-3) sonicating the milled polar carbon nanotube.

In another general aspect, an organic dye is separated from a mixture ofa salt and an organic dye using the separator.

In another general aspect, a polar carbon nanotube dispersion includes apolar carbon nanotube having a three-dimensional porous scaffoldstructure.

In the polar carbon nanotube dispersion, the polar carbon nanotube mayhave an oxygen/carbon atomic ratio of 0.1 or more and a full width athalf maximum of 2.0 or more at a position of 20=25.6° of XRD, and thepolar carbon nanotube dispersion may have viscoelasticity.

In still another general aspect, a method for preparing a polar carbonnanotube dispersion includes: (a) partially oxidizing a carbon nanotubeto prepare a polar carbon nanotube; (b) mechanically milling the polarcarbon nanotube; (c) mixing the milled polar carbon nanotube with asolvent to prepare a dispersion; and (d) sonicating the dispersion.

In the method for preparing a polar carbon nanotube dispersion of thepresent invention, the solvent may be selected from the group consistingof water, ethanol, isopropyl alcohol (IPA), acetone, dimethylformamide(DMF), and N-methylpyrrolidone (NMP), a concentration of the polarcarbon nanotube in the polar carbon nanotube dispersion may be 1 mg/mLor more, and the polar carbon nanotube dispersion may includesubstantially no surfactant.

Other features and aspects will be apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a method for manufacturing a separatorand a separator manufactured therefrom according to the presentinvention.

FIGS. 2(a)-(c) are drawings of structures of (a) MWCNT, (b) FCNT, and(c) GONR, observed by SEM and TEM.

FIG. 3 is a drawing of a cross section of the separator according to anexemplary embodiment of the present invention, observed by SEM.

FIG. 4 is a graph showing XRD patterns of the separator according to anexemplary embodiment of the present invention.

FIGS. 5(a) and (b) are graphs of chemical structures of (a) aconventional oxidized carbon nanotube and (b) a polar carbon nanotube ofthe present invention, observed using an XPS spectrum.

FIG. 6 is a graph showing a BET isotherm of MWCNT, FCNT, and GONR.

FIG. 7 is a drawing of a FCNT scaffold structure which is structuralizedin a high-concentration FCNT dispersion, observed by SEM and TEM.

FIG. 8 is a schematic diagram showing a self-assembly process of FCNT ina dispersion.

FIG. 9 is a schematic diagram showing a three-dimensional porousscaffold structure of FCNT in a dispersion.

FIG. 10 is a drawing showing a dispersion degree when MWCNT and FCNTwere dissolved in various solvents.

FIGS. 11(a)-(c) are drawings of (a) an upper surface of a FCNT filmobserved by SEM, (b) the FCNT film observed by AFM, and (c) a crosssection of the FCNT film observed by SEM.

FIGS. 12(a) and (b) are graphs showing permeability and rejection rates(a) when a salt was permeated through the separator according to anexemplary embodiment of the present invention and (b) when an organicdye was permeated through the separator according to an exemplaryembodiment of the present invention.

FIG. 13 is a graph showing permeability and rejection rates whenpressure was applied to the separator according to an exemplaryembodiment of the present invention.

FIG. 14 is a graph showing permeability and rejection rates when amixture of a salt and an organic dye was permeated through the separatoraccording to an exemplary embodiment of the present invention.

FIG. 15 is a graph showing separation factors when a mixture of a saltand an organic dye was permeated through the separator according to anexemplary embodiment of the present invention.

DESCRIPTION OF THE INVENTION

Hereinafter, a polar carbon nanotube dispersion, a separator includingthe same, and a method for manufacturing the same of the presentinvention will be described in detail with reference to the accompanyingdrawings.

The drawings to be provided below are provided by way of example so thatthe spirit of the present invention can be sufficiently transferred to aperson skilled in the art to which the present invention pertains.Therefore, the present invention is not limited to the drawings providedbelow but may be embodied in many different forms, and the drawingssuggested below may be exaggerated in order to clear the spirit of thepresent invention.

Technical terms and scientific terms used herein have the generalmeaning understood by those skilled in the art to which the presentinvention pertains, unless otherwise defined, and the description forthe known function and configuration which may unnecessarily obscure thegist of the present invention will be omitted in the followingdescription and the accompanying drawings.

In addition, the singular form used in the specification and claimsappended thereto may be intended to include a plural form also, unlessotherwise indicated in the context.

In the present specification and the appended claims, the terms such as“first” and “second” are not used in a limited meaning but used for thepurpose of distinguishing one constituent element from other constituentelements.

In the present specification and the appended claims, the terms such“comprise” or “have” mean that there is a characteristic, or aconstituent element described in the specification, and as long as it isnot particularly limited, a possibility of adding one or more othercharacteristics or constituent elements is not excluded in advance.

In the present specification and the appended claims, when a portionsuch as a membrane (layer), a region, and a constituent element ispresent on another portion, not only a case in which the portion is incontact with and directly on another portion but also a case in whichother membranes (layers), other regions, other constitutional elementsare interposed between the portions is included.

The term “micropores” in the present invention means that internal poreshave an average diameter of less than 2 nm, “mesopores” means thatinternal pores have an average diameter of 2 nm to 50 nm, and“macropores” means that internal pores have an average diameter of morethan 50 nm.

The separator according to the present invention is characterized byincluding: a porous support; a porous first coating layer including apolar carbon nanotube, placed on the porous support; and a secondcoating layer including a polar one-dimensional carbon body, placed onthe porous first coating layer.

The porous support according to an exemplary embodiment of the presentinvention is not particularly limited as long as it is a material usedas a support of the separator, but as a non-limiting example, it may bea porous inorganic support or a porous polymer support, preferably aporous polymer support. The porous polymer support may be a naturalpolymer or a synthetic polymer, but is not limited to a certain polymer.The natural polymer may be a cellulose-based polymer or a derivativethereof, and a non-limiting example of the synthetic polymer may beselected from polycarbonate-based polymers, polyamide-based polymer,polyimide-based polymers, polyolefin-based polymers, polyacrylate-basedpolymers, polysulfone-based polymers, polyether-based polymers,polyester-based polymers, and the like. A specific example thereof maybe selected from the group consisting of polyethylene, polypropylene,polybutylene, polypentene, polymethylpentene, polyethyleneterephthalate, polybutylene terephthalate, polyacetal, polyethersulfone,polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene,and copolymers thereof, and preferably, polyethersulfone (PES) may beused.

The porous support according to an exemplary embodiment of the presentinvention may have a pore average size of 0.01 μm to 1 μm, preferably0.05 μm to 0.5 μm, and more preferably 0.1 μm to 0.3 μm. In addition,the porous support according to an exemplary embodiment of the presentinvention may have a porosity of 20% to 80%, preferably 20% to 60%. Byhaving the pore size and the porosity in the above ranges, the materialadsorbed to the separator may be easily permeated through the support.

The polar carbon nanotube according to an exemplary embodiment of thepresent invention may include an oxygen-containing functional group, andthe oxygen-containing functional group may be any one or more selectedfrom the group consisting of a hydroxyl group, an epoxy group, and acarboxyl group. In addition, the polar carbon nanotube according to anexemplary embodiment of the present invention may have an entangledstructure. By including the functional group and the entangledstructure, the polar carbon nanotube is stably placed on the poroussupport, so that it is not easily released from the porous support evenat high pressure.

The polar carbon nanotube according to an exemplary embodiment of thepresent invention may have an oxygen/carbon atomic ratio of 0.1 or more,preferably 0.1 to 0.5, and more preferably 0.2 to 0.3.

In addition, the polar carbon nanotube according to an exemplaryembodiment of the present invention may have a larger full width at halfmaximum (FWHM) at a position of 20=25.6° of XRD than the full width athalf maximum of a common carbon nanotube (1.98), and the full width athalf maximum may be specifically 2.0 or more, preferably 2.2 or more,and more preferably 2.2 to 2.5. By having the oxygen/carbon atomic ratioand FWHM in the above ranges, it contains a plurality of oxygen groups,so that a strong electrostatic repulsion occurs in a solution, and athree-dimensional scaffold structure may be formed in a solvent byself-assembly rather than a simple agglomeration is formed. However,since the carbon nanotube to which a polar group is introduced by acommon oxidation reaction has a low oxygen/carbon atomic ratio, it hasimproved dispersibility in an aqueous phase, but has a high tendency toagglomerate, and thus, it is impossible to prepare a homogeneoushigh-concentration dispersion. Here, the scaffold structure means aporous structure like a sponge which is self-entangled by self-assembly,and the size of scaffold pores corresponds to usual macropores having adiameter of several micrometers. The polar carbon nanotube forming thescaffold forms an entangled structure.

The polar carbon nanotube according to an exemplary embodiment of thepresent invention may be prepared by applying a viscoelastic polarcarbon nanotube dispersion. Since the polar carbon nanotube prepared byapplying a viscoelastic polar carbon nanotube dispersion has a lowsurface roughness and a viscoelasticity like a polymer solution, acoating solution may be uniformly applied to a substrate by barcoating/doctor blade coating.

The polar carbon nanotube dispersion according to an exemplaryembodiment of the present invention may form a structure having athree-dimensional porous scaffold, and the three-dimensional porousscaffold structure may be formed by a structure in which polar carbonnanotubes are entangled with each other. Since the polar carbon nanotubepresent in the polar carbon nanotube dispersion has a three-dimensionalporous scaffold structure, agglomeration of polar carbon nanotubes maybe suppressed, and the carbon nanotubes may be dispersed in varioussolvents without addition of a surfactant.

The polar carbon nanotube according to an exemplary embodiment of thepresent invention may have a surface area of 100 to 500 m²/g, preferably200 to 300 m²/g. By having the surface area in the above range,agglomeration of the polar carbon nanotubes in the first coating layermay be suppressed, and the carbon nanotubes may be dispersed in varioussolvents without addition of a surfactant.

The polar group of the polar carbon nanotube according to an exemplaryembodiment of the present invention may be positioned on the surface ofthe carbon nanotube. The carbon nanotube including a polar grouppositioned on the surface has both hydrophilicity by a polar group andhydrophobicity by carbon, thereby forming a uniform coating layerwithout much dependence on the surface properties of the porous support.

The polar carbon nanotube according to an exemplary embodiment of thepresent invention may be a single-walled polar carbon nanotube or amulti-walled polar carbon nanotube, and preferably a multi-walled carbonnanotube may be used. The multi-walled polar carbon nanotube hasexcellent mechanical strength, excellent structural maintenance totensile repeatability, and a large tensile range, and thus, isadvantageous for use in a separator. In addition, the polar carbonnanotube may have an aspect ratio of 100 to 50,000, preferably 1,000 to45,000, and more preferably 4,000 to 40,000.

By having the aspect ratio in the above range, mechanical strength maybe excellent without collapse of the structure of the scaffold on theporous support.

The polar one-dimensional carbon body according to an exemplaryembodiment of the present invention may include a polar group, and thepolar group may include any one or more selected from the groupconsisting of a hydroxyl group, an epoxy group, and a carboxyl group. Inaddition, the polar group may be unevenly distributed at the edge of thepolar one-dimensional carbon body, and a ratio of a heteroatom containedin the polar group of the total atoms of the polar one-dimensionalcarbon body may be 0.1 to 50 atom %, preferably 0.1 to 20 atom %, andmore preferably 1 to 10 atom %, but is not limited thereto.

Since the polar one-dimensional carbon body according to an exemplaryembodiment of the present invention may include a polar group to have acertain level of hydrophilicity, it has excellent dispersibility towater and may form a uniform aqueous dispersion or aqueous solution, andthe aqueous dispersion or aqueous solution may be applied to easily forma uniform coating layer. In addition, the polar one-dimensional carbonbody including the polar group unevenly distributed at the edge may haveboth a certain level of hydrophilicity by the polar group andhydrophobicity by carbon, thereby forming a uniform coating layer.

The polar one-dimensional carbon body according to an exemplaryembodiment of the present invention may be a graphene-based compound,preferably a graphene nanoribbon. The graphene nanoribbon is preferredfor being uniformly coated on the polar carbon nanotube to form a thinfilm, and since the graphene nanoribbon forms a thin film stablydeposited on the polar carbon nanotube by interaction, and also, bothpermeability and selectivity depending on the size of particles may beimplemented.

The graphene nanoribbon according to an exemplary embodiment of thepresent invention may have a width length of 10 to 100 nm, preferably 20to 70 nm, and more preferably 30 to 50 nm. By having the width length inthe above range, the graphene nanoribbon may have an excellent surfacearea. In addition, the graphene nanoribbon may have an aspect ratio of 1to 50,000, preferably 10 to 40,000. By having the aspect ratio in theabove range, dispersibility may be excellent, and also, mechanicalstrength may be excellent.

The second coating layer according to an exemplary embodiment of thepresent invention may have a multilayer structure. Here, an interlayerspacing of the multilayer structure may be formed to be 1 to 100 Å,preferably 3 to 50 Å, and more preferably 5 to 20 Å.

By having the interlayer spacing in the above range, permeation ofmaterials having a relatively large size is prevented without preventingpermeation of small materials, thereby implementing both excellentpermeability and selectivity.

A thickness ratio between the porous first coating layer and the secondcoating layer according to an exemplary embodiment of the presentinvention may be 5:1 to 20:1, preferably 10:1 to 20:1. Morespecifically, the porous first coating layer according to an exemplaryembodiment of the present invention may be formed to have a thickness of1 to 10 μm, preferably 1.5 to 7.5 μm, and more preferably 2 to 5 μm. Inaddition, the second coating layer according to an exemplary embodimentof the present invention may be formed to have a thickness of 10 to 500nm, preferably 50 to 400 nm, and more preferably 100 to 300 nm. Byhaving the thickness in the above range, the stability, thepermeability, and the selectivity of the separator may be allimplemented.

The polar carbon nanotube according to an exemplary embodiment of thepresent invention may have a lower oxygen/carbon atomic ratio than thepolar one-dimensional carbon body.

More specifically, the polar one-dimensional carbon body according to anexemplary embodiment of the present invention may have an oxygen/carbonatomic ratio of 0.2 or more, preferably 0.2 to 0.5, and more preferably0.3 to 0.4. That is, the polar carbon nanotube may have a loweroxidation degree than the polar one-dimensional carbon body. Due to thedifference in the oxidation degree, the polar one-dimensional carbonbody coating layer has a denser structure than the polar carbonnanotube, so that the permeability may vary with the particle size, andalso, the porous support, the polar carbon nanotube, and the polarone-dimensional carbon body coating layer may be stably bonded. Inaddition, since both the porous first coating layer and the secondcoating layer include a polar group, specifically, an oxygen-containingfunctional group, a n-n bond may be formed by a hydrogen bond betweenpolar groups and an aromatic n electron. Therefore, the porous firstcoating layer and the second coating layer may be stably bonded by thehydrogen bond and the n-n bond.

The separator according to an exemplary embodiment of the presentinvention may have an organic dye rejection rate of 80% or more and asodium chloride rejection rate of 20% or less, preferably an organic dyerejection rate of 85% or more and a sodium chloride rejection rate of15% or less, with respect to a mixture of sodium chloride and theorganic dye under the conditions of 10 bar.

Therefore, a method for separating an organic dye, which ischaracterized by separating an organic dye from a mixture of a salt andthe organic dye, using the separator according to an exemplaryembodiment of the present invention, may be provided.

Here, the salt is not particularly limited as long as it is commonlyused, but as a non-limiting example, Na₂SO₄, NaCl, MgSO₄, MgCl₂, and thelike may be selected, and the organic dye is also not particularlylimited as long as it is a commonly used organic dye, but as anon-limiting example, methyl red (MR), methylene blue (MnB), brilliantblue G (BBG), rose bengal (RB), and the like may be selected.

Hydrogen may be separated from a gas mixture using the separatoraccording to an exemplary embodiment of the present invention, in whichthe gas mixture may include hydrocarbon and hydrogen, and specifically,the hydrocarbon according to the present invention may be C1 to C8aliphatic hydrocarbon, and for example, may be selected from CH₄, C₂H₄,C₂H₆, C₃H₆, C₃H₈, and the like, but the present invention is not limitedthereto. In addition, since a dye may be separated from dye waste water,the separator may be used as a separator for treating dye waste water.

The method for manufacturing a separator according to the presentinvention is characterized by including: (a) preparing a dispersionincluding a polar carbon nanotube; (b) preparing a dispersion includinga polar one-dimensional carbon body; (c) applying the dispersionincluding a polar carbon nanotube on a porous support to prepare aporous first coating layer; and (d) applying the dispersion including apolar one-dimensional carbon body on the porous first coating layer toprepare a second coating layer.

In the method for manufacturing a separator according to an exemplaryembodiment of the present invention, the step (a) of preparing adispersion including a polar carbon nanotube may include (a-1) partiallyoxidizing a carbon nanotube to prepare the polar carbon nanotube; (a-2)mechanically milling the polar carbon nanotube; and (a-3) sonicating themilled polar carbon nanotube.

Here, by the step of mechanically milling the polar carbon nanotube andthe step of sonicating, the agglomeration of the polar carbon nanotubeis prevented, and the porous first coating layer is uniformly coated onthe porous support to prevent exfoliation.

The polar carbon nanotube dispersion according to the present inventionis characterized by including a polar carbon nanotube having athree-dimensional porous scaffold structure.

In the polar carbon nanotube dispersion according to the presentinvention, the polar carbon nanotube may be a polar carbon nanotubeincluded in the separator as described above. Therefore, the polarcarbon nanotube in the polar carbon nanotube dispersion according to thepresent invention may have an oxygen/carbon atomic ratio of 0.1 or more,preferably 0.1 to 0.5, and more preferably 0.2 to 0.3. In addition, thepolar carbon nanotube according to an exemplary embodiment of thepresent invention may have a larger full width at half maximum (FWHM) ata position of 20=25.6° of XRD than the full width at half maximum of acommon carbon nanotube (1.98), and the full width at half maximum may bespecifically 2.0 or more, preferably 2.2 or more, and more preferably2.2 to 2.5. In addition, the polar carbon nanotube dispersion may haveviscoelasticity. By the characteristics, a three-dimensional scaffoldstructure may be formed in a solvent by self-assembly, and ahigh-concentration dispersion which is homogeneous in various solventsas compared with common carbon nanotubes having a high tendency ofagglomeration may be prepared.

The method for preparing a polar carbon nanotube dispersion according tothe present invention is characterized by including: (a) partiallyoxidizing a carbon nanotube to prepare a polar carbon nanotube; (b)mechanically milling the polar carbon nanotube; (c) mixing the milledpolar carbon nanotube with a solvent to prepare a dispersion; and (d)sonicating the dispersion.

In the method for preparing a polar carbon nanotube dispersion accordingto an exemplary embodiment of the present invention, the solvent of thedispersion is not particularly limited as long as it may disperse polarcarbon nanotubes, but may be water, ethanol, 2-propanol, acetone,dimethylformamide, or N-methyl-2-pyrrolidone, and water is preferred.

In the method for preparing a polar carbon nanotube dispersion accordingto an exemplary embodiment of the present invention, a concentration ofthe polar carbon nanotube in the dispersion including the polar carbonnanotube may be 1 mg/mL or more, preferably 1 to 100 mg/mL, and morepreferably 20 to 50 mg/mL. By having the concentration in the aboverange, a conventional problem of only allowing dispersion at a lowconcentration and showing agglomerated particles at a high concentrationis solved, and that excellent dispersibility may be shown.

In the method for preparing a polar carbon nanotube dispersion accordingto an exemplary embodiment of the present invention, the polar carbonnanotube dispersion may include substantially no surfactant. Here,including substantially no surfactant means that the weight of thesurfactant is included at less than 0.1 wt %, specifically at less than0.01 wt %, with respect to the total weight of the polar carbon nanotubedispersion.

Generally, in order to manufacture a polar carbon nanotube film, thedispersion may be prepared using a surfactant, but in this case, theprocess is complicated and additional washing is needed for removing thesurfactant. In addition, in order not to use the surfactant, a solventsuch as dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP) is usedto prepare a dispersion, but the properties of the solvent cause theporous support to be damaged. Therefore, in the method for preparing apolar carbon nanotube dispersion according to an exemplary embodiment ofthe present invention, the above problem may be solved by includingsubstantially no surfactant.

Hereinafter, the present invention will be described in detail by theexamples. However, the examples are for describing the present inventionin more detail, and the scope of the present invention is not limited tothe following examples.

<Example> GONR/FCNT/PES Separators

A polar carbon nanotube (functionalized carbon nanotube; FCNT) andgraphene oxide nanoribbon (GONR) were prepared by adjusting theoxidation degree of a multi-walled carbon nanotube (MWCNT) by changing aratio of carbon nanotube (CNT)/KMnO₄ and an oxidation time. Anoxygen-containing functional group was produced in MWCNT by a reactionof KMnO₄ to β and γ-alkene, and by increasing the density of theoxygen-containing functional group, compression of MWCNT was released. AFCNT layer was applied to polyethersulfone (PES), and a GONR layer wassequentially coated by vacuum filtration, thereby manufacturing a doublelayer including FCNT and GONR layers. Specific manufacturing methods areas follows:

1. Preparation of FCNT and GONR

First, FCNT was prepared by partial oxidation of MWCNT. MWCNT had adiameter of 15 to 25 nm, a length of 20 to 100 μm, and a thickness of 7to 12 layers. 200 mL of H₂SO₄ was mixed with 4 g of MWCNT, 8 g of KMnO₄was added to the mixture in an ice bath, and then the mixture wasstirred at a temperature of 35° C. After stirring for 1 hour, 350 mL ofdeionized water and 80 mL of hydrogen peroxide (H₂O₂) were added to themixture sequentially to finish the oxidation reaction, and the remainingoxidizing agent was removed. Next, GONR was synthesized in the samemanner as the preparation method of FCNT, except that 2 g of MWCNT and10 g of KMnO₄ were mixed. Thereafter, the mixed solution was filtratedthrough a cellulose filter paper to remove impurities and an acidicsolvent, thereby obtaining FCNT and GONR.

2. Preparation of FCNT Dispersion

FCNT was ball-milled at 1200 rpm for 6 hours and then ball-milled at 300rpm for 6 hours. After the ball milling, deionized water was added toFCNT to adjust the concentration to 40 mg/mL, the prepared dispersionwas sonicated for 3 hours by a horn sonicator (VC 505, Sonics &Materials, USA). A FCNT dispersion showed viscosity like hydrogel afterthe sonication. The FCNT dispersion was further diluted with deionizedwater to prepare a 1 mg/mL of a solution.

3. Preparation of GONR/FCNT/PES Separator

A PES support (pore size: 0.2 μm, diameter: 5 cm) was coated with 0.5 mLof a FCNT dispersion (40 mg/mL) and was dried at 60° C. for 3 hours. Adiluted GONR solution was filtered and a FCNT layer was coated with aGONR layer. 0.1 mL of a GONR dispersion (5 mg/mL) was diluted with 100mL of deionized water, filtered under vacuum using a FCNT-coated PESsupport, and then dried at 60° C. for 3 hours.

<Comparative Example 1> FCNT/PES Separator

The separator was manufactured in the same manner as in Example, exceptthat the GONR layer was not coated in step 3.

<Comparative Example 2> GONR/PES Separator

The separator was manufactured in the same manner as in Example, exceptthat the FCNT layer was not coated in step 3.

<Experimental Example 1> Structure Analysis

FIG. 1 is a schematic diagram of the method for manufacturing theGONR/FCNT/PES separator according to the example and the GONR/FCNT/PESseparator manufactured therefrom. It is recognized that a FCNT layer wascoated on a polyethersulfone (PES) support and a GONR coating layer wascoated on the FCNT layer.

Next, the form and structure of MWCNT, FCNT, and GONR were observedusing a transmission electron microscope TEM; JEM-F200, JEOL, Japan) anda field emission scanning electron microscope (SEM; 7610f-plus, JEOL,Japan).

Referring to FIG. 2(a), a MWCNT bundle having macropores may be observedin a SEM image of MWCNT, and the diameter of MWCNT was confirmed to be15 to 25 nm. Next, referring to FIG. 2(b), it is observed that FCNTprepared by partially oxidizing MWCNT maintained a multiwall structureand a porous network like MWCNT, and also the partially oxidized regionis indicated with an arrow. Next, referring to FIG. 2(c), in GONRprepared by unzipping MWCNT, it was confirmed that the width of ananoribbon was widened to 30 to 50 nm, and there was a dense networkformed of GONR.

Next, the cross section of the GONR/FCNT/PES separator according to theexample was observed by a SEM image. Referring to FIG. 3 , it wasconfirmed that a FCNT layer having a thickness of 3 μm was applied on aPES support layer, and the thickness of the GONR layer on the upper sidewas about 200 nm. More specifically, the GONR layer formed a dense layeras compared with the FCNT layer having a porous structure so that theGONR layer may act as a selective layer. Exfoliation of GONR and FCNTwas not observed even after the double membrane was dipped in a solventincluding water, ethanol, isopropyl alcohol (IPA), sulfuric acid(H₂SO₄), and it was confirmed that sodium hydroxide (NaOH), and PES,GONR, and FCNT layers were stably coated.

Next, since the interlayer spacing of the laminated GONR layer isimportant to determine a separation ability, the XRD pattern of theGONR/FCNT/PES separator was investigated as compared with MWCNT, FCNT,and GONR. For calculating the interlayer spacing of the GONR/FCNT/PESseparator, observation was performed at a scanning speed of 1° C./min byhigh resolution XRD (Smartlab, Rigaku, Japan).

Referring to FIG. 4 , FCNT and MWCNT had a peak at 25.6° to showsimilarity in the XRD pattern, and it was confirmed therefrom that theinterlayer spacing and the structure were not influenced by partialoxidation from MWCNT to FCNT, so that they had a similar structure andhad an oxygen-containing functional group produced on the surface.However, it was confirmed that FCNT had a broadened peak and had adecreased peak strength as compared with MWCNT due to the decreasedcrystallinity by oxidation of MWCNT.

However, in the XRD pattern of GONR, the peak was showed at 11.8°, andit is recognized therefrom that there was an oxygen-containingfunctional group in the base surface and the edge and the interlayerspacing was widened to about 7.5 Å. In addition, the FCNT/PES separatoraccording to Comparative Example 1 and the GONR/FCNT/PES separatoraccording to the example showed a peak at 25.6° by the FCNT layer and afurther peak was observed at 11.8° in the GONR/FCNT/PES separator by theGONR layer. That is, it was confirmed that the FCNT layer did not affectthe lamination of the GONR layer.

Next, an XPS spectrum was observed for measuring an oxidation degree ofFCNT. Referring to FIG. 5(a), a surface treatment was minimized inconventional oxidized CNT for structural maintenance and dispersion ofindividualized CNT, and thus, the oxidation degree was very low.However, referring to FIG. 5(b), considering that the structure of CNTwas maintained while a very high O 1 s peak was also shown, in FCNT, itwas confirmed that FCNT was more oxidized than conventional oxidized CNTto have a large number of oxygen functional groups.

Next, the porous structures of MWCNT, FCNT, and GONR were observed usinga BET isotherm. Referring to FIG. 6 , a layer containing FCNT (215.3m²/g) had a larger surface area than a layer containing MWCNT (178.8m²/g), and this is because nitrogen gas may be adsorbed/desorbed on theinternal wall by internal surface exposure by partial oxidation ofMWCNT. It was confirmed that the layer containing GONR (255.6 m²/g) hada larger surface area. It was confirmed therefrom that FCNT was oxidizedwhile maintaining the structure of MWCNT, and the GONR layer had smallpores being densely packed to have a denser structure than the FCNTlayer. Next, the scaffold structure of FCNT was observed using a fieldemission scanning electron microscope (SEM; 7610f-plus, JEOL, Japan).Referring to FIG. 7 , the scaffold structure of FCNT was observed, andit was confirmed therefrom that a three-dimensional scaffold structurewas produced when MWCNT was partially oxidized to FCNT. It was confirmedthat FCNT entangled at a high concentration formed a nanoplate and athree-dimensional scaffold. That is, it was confirmed that a strongrepulsion to each other occurred in FCNT by CNT having a high aspectratio (4000-40000) and many oxygen functional groups, and athree-dimensional scaffold structure by self-assembly was formed ratherthan simple agglomeration or bundle by a mechanical mixing process suchas ball milling and sonication.

<Experimental Example 2> Evaluation of Dispersion Degree of FCNTDispersion

FIGS. 8 and 9 show the schematic diagrams of the self-assembly processof FCNT in the dispersion and FCNT having a three-dimensional porousscaffold structure. When conventional CNT is dispersed in a solvent, itis present in the state of agglomerated CNT bundles, but FCNT of thepresent invention was confirmed to have a three-dimensional porousscaffold structure when dispersed in a solvent at a high concentration.That is, it was confirmed that solvents were present between entangledFCNT and on the outside of the three-dimensional porous scaffold, andthus, FCNT had dispersion stability even at a high concentration.

Next, in order to evaluate the dispersion degree of the prepared FCNTdispersion, the prepared CNT was dissolved in various solvents andobserved.

Referring to FIG. 10 , when conventional MWCNT and FCNT obtained bypartial oxidation of MWCNT were mechanically mixed by ball milling andsonication and were dispersed in various solvents at a concentration of1 mg/mL, MWCNT was not dispersed to show agglomeration of particles atthe same concentration, while the phenomenon was not observed in FCNT. Adispersion which was dispersed in water at a high concentration of 40mg/mL or more may be prepared based on the dispersion stability, and thedispersion had viscoelasticity. That is, it was confirmed thatagglomeration was suppressed regardless of the kind of solvents, by FCNThaving a three-dimensional scaffold structure produced by partialoxidation of MWCNT, and thus, the dispersion degree was greatlyimproved.

Next, it was confirmed that a very uniform FCNT film was manufactured bybar coating which is a kind of manufacturing process of a film having alarge area using a high-concentration FCNT dispersion havingviscoelasticity. Referring to FIGS. 11(a) and (b), no agglomeration wasobserved in the manufactured FCNT film, and it is recognized that thefilms had very low R_(q) of 15.6 nm and a very uniform structure. Inaddition, referring to FIG. 11(c), it was confirmed that themanufactured film had a thickness of about 8.5 μm and had a porousstructure of CNT by the side structure of the film. It was confirmedthat as a force was applied to the FCNT scaffold, the scaffold wasbroken and the nanoplates of the scaffold were arranged and coated,thereby manufacturing the uniform film.

<Experimental Example 3> Filtration Test Using GONR/FCNT/PES Separator

In order to evaluate the filtration performance of the GONR/FCNT/PESseparator according to the example, a salt and a dye solution weremeasured using dead-end filtration at 10 bar. A filtration test wasperformed using 0.1 M Na₂SO₄, NaCl, MgSO₄, and MgCl₂ salts and 10 mg/Lof methyl red (MR, 269 Da), methylene blue (MnB, 320 Da), brilliant blueG (BBG, 854 Da), and rose bengal (RB, 1018 Da) organic dyes as a probemolecule. An effective area of the membrane was 4.52 cm², andpressurization by N₂ was performed and filtration was performed 10 bar.For high-pressure performance evaluation, a gas pressure was increasedfrom 10 bar to 30 bar using a regulator. The concentration of the saltwas measured using an ion probe conductor (Hi 9033, Hanna Instruments,USA), and the concentration of the organic dye was measured by UV-visspectrometry.

Here, permeability (J) was calculated by the following Equation 1:

$\begin{matrix}{J = \frac{V_{p}}{t \times A \times \Delta p}} & \left\lbrack {{Equation}1} \right\rbrack\end{matrix}$

wherein V_(p) is a volume (L) of a permeated liquid, A is an effectivearea (m²) of a membrane, t is a permeation time (h), and Op is anintermembrane differential pressure (bar).

In addition, a rejection rate (R) was calculated by the followingEquation 2:

$\begin{matrix}{R = {\frac{\left( {C_{f} - C_{p}} \right)}{C_{f}} \times 100}} & \left\lbrack {{Equation}2} \right\rbrack\end{matrix}$

wherein C_(f) is a concentration of a supply solution, and C_(p) is aconcentration of a permeated solution.

Next, membrane performance under real conditions was measured using across flow filtration system. 10 mg/L BBG was used as a probe molecule,and an intermembrane pressure was 8 bar. Equations 1 and 2 were used tocalculate the permeability and the rejection rate, and the effectivearea of the membrane was 7.07 cm².

First, the permeability and the rejection rate of Na₂SO₄ (Z⁺/Z⁻: ½),NaCl (Z⁺/Z⁻: 1), MgSO₄ (Z⁺/Z⁻: 1), and MgCl₂ (Z⁺/Z⁻: 2) having aconcentration of 0.1 M were measured. Referring to FIG. 12(a),permeability in the range of 7 to 11 LMH/bar was shown in all saltsolutions. However, since the d-spacing (7.5 Å) of the GONR layer waslarger than the radius of hydrated ions (Na⁺: 3.79 Å, Mg²⁺: 4.28 Å, Cl⁻:3.32 Å, SO₄ ²⁻: 3.79 Å), the rejection rate was confirmed to be lessthan 20%.

In addition, the permeability and the rejection rate were measured usingmethyl red (MR, 269 Da, electrically neutral), methylene blue (MnB, 320Da, positive charge), brilliant blue G (BBG, 854 Da, negative charge),and rose bengal (RB, 1018 Da, negative charge) as a probe molecule.Referring to FIG. 12(b), high rejection rates (˜100%) were shown in alldyes, and since rich nanopores were present in the GONR layer, a purepermeability (22 LMH/bar) of 86% was maintained in the presence of a dyemolecule (19 LMH/bar). That is, it was confirmed that due to thepresence of the GONR layer, an organic dye having a large particle sizeshowed a high rejection rate by the double membrane.

Next, in order to measure mechanical stability at high pressure, 10 mg/Lof a BBG solution was used to increase the pressure up to 30 bar,thereby performing a filtration test. Referring to FIG. 13 , as anoperation pressure was increased from 11 bar to 30 bar, a flow rate wasincreased from 228 LMH to 467 LMH, and stable mechanical propertiesprovided by the FCNT layer were confirmed while maintaining a rejectionrate of 100%. That is, it was confirmed that mechanical stability wasmaintained by the entangled structure of the FCNT layer and the stablebond between the FCNT layer and the GONR layer even at increasedpressure.

<Experimental Example 4> Diafiltration Test Using GONR/FCNT/PESSeparator

The GONR/FCNT/PES separator according to the example was used to measurethe separation ability of a salt/dye mixture. The mixture was preparedby dispersing a dye (MR, MnB, BBG) having a concentration of 10 mg/L ina 6 g/L NaCl solution. The moisture permeability and the rejection ratewere determined by Equations 1 and 2, and the separation factor of thediafiltration was calculated using the following Equation 3:

$\begin{matrix}{{{Separation}{Factor}} = \frac{1 - R_{NaCl}}{1 - R_{dye}}} & \left\lbrack {{Equation}3} \right\rbrack\end{matrix}$

First, diafiltration performance was measured using BBG/NaCl, MnB/NaCl,and MR/NaCl in a dead-end system at a pressure of 10 bar. The mixtureswere prepared at a concentration of 10 mg/L and 6 g/L, respectively forthe dye and the salt. Referring to FIG. 14 , the rejection rates of98.4%, 95.3%, and 87.7% were shown, respectively, for BBG, MnB, and MR,in each dye/salt mixture. That is, the rejection rate in the presence ofa salt ion was a little decreased as compared with the case without asalt ion, and this is confirmed to be because the hydrated radius of thedye was decreased by a salting out effect by the presence of the salt,and the interlayer spacing of GONR by the inserted ion was increased.However, it was confirmed that due to the presence of the GONR layer, anorganic dye having a large particle size showed a high rejection rate bythe double membrane, in both cases of using the dead-end system andusing a real double membrane.

Next, in order to evaluate the separation ability of the salt/dyemixture in the dead-end system, a diafiltration separation factor wasmeasured. Referring to FIG. 15 , an ideal separation factors of theGONR/FCNT/PES separator were 853, 122, and 95, respectively, forBBG/NaCl, MnB/NaCl, and MR/NaCl mixtures, but the real separationfactors of the GONR/FCNT/PES separator was 55.6, 18.5, and 6.5, whichshowed lower diafiltration performance than expected. However, thediafiltration performance of the separator was higher than the GONRmembrane of which the separation factors were 7.5, 3.3, and 1.7. It wasconfirmed that this is due to the presence of the FCNT layer whichsuppresses the change of the GONR layer structure under high pressureand an ion solution.

In summary, according to the separator and the method for manufacturingthe same of the present invention, a separator having excellentmechanical stability is manufactured without an additional chemicaltreatment and chemical crosslinking, and thus, a separator havingexcellent separation ability even at high pressure may be manufactured.

According to the polar carbon nanotube dispersion, the separatormanufactured therefrom, and the method for manufacturing the same of thepresent invention, a polar carbon nanotube dispersion, which may bedispersed in a solvent at a high concentration even without use of asurfactant or a stabilizer, may be prepared, and a separator, which isnot easily exfoliated and may be stably used even under a high pressure,may be manufactured, based on a polar carbon nanotube prepared from thepolar carbon nanotube dispersion and a polar one-dimensional carbonbody.

1. A separator comprising: a porous support; a porous first coating layer including a polar carbon nanotube, placed on the porous support; and a second coating layer including a polar one-dimensional carbon body, placed on the porous first coating layer.
 2. The separator of claim 1, wherein the polar carbon nanotube has an oxygen/carbon atomic ratio of 0.1 or more and a full width at half maximum of 2.0 or more at a position of 2θ=25.6° of XRD.
 3. The separator of claim 1, wherein the porous first coating layer is prepared by applying a viscoelastic polar carbon nanotube dispersion.
 4. The separator of claim 1, wherein a polar group of the polar carbon nanotube is positioned on a surface of the carbon nanotube.
 5. The separator of claim 1, wherein a polar group of the polar one-dimensional carbon body includes any one or more selected from the group consisting of a hydroxyl group, an epoxy group, and a carboxyl group.
 6. The separator of claim 5, wherein the polar group is unevenly distributed at an edge of the polar one-dimensional carbon body.
 7. The separator of claim 1, wherein the polar one-dimensional carbon body is a graphene nanoribbon.
 8. The separator of claim 1, wherein an interlayer spacing of the second coating layer is 5 to 20 Å.
 9. The separator of claim 1, wherein a thickness ratio between the porous first coating layer and the second coating layer is 5:1 to 20:1.
 10. The separator of claim 1, wherein the polar carbon nanotube has a lower oxygen/carbon atomic ratio than the polar one-dimensional carbon body.
 11. The separator of claim 1, wherein an organic dye rejection rate is 80% or more and a sodium chloride rejection rate is 20% or less, with respect to a mixture including sodium chloride and an organic dye, under a condition of 10 bar.
 12. A method for manufacturing a separator, the method comprising the steps of: (a) preparing a dispersion including a polar carbon nanotube; (b) preparing a dispersion including a polar one-dimensional carbon body; (c) applying the dispersion including a polar carbon nanotube on a porous support to prepare a porous first coating layer; and (d) applying the dispersion including a polar one-dimensional carbon body on the porous first coating layer to prepare a second coating layer.
 13. The method for manufacturing a separator of claim 12, wherein the preparing of a dispersion including a polar carbon nanotube in step (a) includes the steps of: (a-1) partially oxidizing a carbon nanotube to prepare the polar carbon nanotube; (a-2) mechanically milling the polar carbon nanotube; and (a-3) sonicating the milled polar carbon nanotube.
 14. A method for separating an organic dye, the method comprising: separating an organic dye from a mixture of a salt and the organic dye using the separator of claim
 1. 15. A polar carbon nanotube dispersion comprising: a polar carbon nanotube having a three-dimensional porous scaffold structure.
 16. The polar carbon nanotube dispersion of claim 15, wherein the polar carbon nanotube has an oxygen/carbon atomic ratio of 0.1 or more and a full width at half maximum of 2.0 or more at a position of 2θ=25.6° of XRD.
 17. The polar carbon nanotube dispersion of claim 15, wherein the polar carbon nanotube dispersion has viscoelasticity.
 18. A method for preparing a polar carbon nanotube dispersion, the method comprising the steps of: (a) partially oxidizing a carbon nanotube to prepare a polar carbon nanotube; (b) mechanically milling the polar carbon nanotube; (c) mixing the milled polar carbon nanotube with a solvent to prepare a dispersion; and (d) sonicating the dispersion.
 19. The method for preparing a polar carbon nanotube dispersion of claim 18, wherein the solvent is selected from the group consisting of water, ethanol, isopropane alcohol (IPA), acetone, dimethylformamide (DMF), and N-methylpyrrolidone (NMP).
 20. The method for preparing a polar carbon nanotube dispersion of claim 18, wherein a concentration of the polar carbon nanotube in the polar carbon nanotube dispersion is 1 mg/mL or more.
 21. The method for preparing a polar carbon nanotube dispersion of claim 18, wherein the polar carbon nanotube dispersion includes substantially no surfactant. 